Connection of metals to ceramics

ABSTRACT

In the connection of a ceramic post to a metal plate, the post is located in a hole in the plate with a porous ring of brazing alloy surrounding the post in contact with the plate. The brazing alloy is then heated to braze the ring to the plate, and the invention resides in the provision of a ring of brazing alloy which has such an initial porosity that when the ring is heated to the brazing temperature the ring sinters to have less porosity and expands less than the post and so will be a tight fit on the post.

United States Patent Inventor Appl. No. Filed Patented Assignee Priority CONNECTION 0F METALS T0 CERAMICS 2 Claims, 2 Drawing Figs.

US. Cl 29/473.l, 29/500, 29/D1G. 35

Int. Cl B231: 31/02 Field of Search 29/447, 473.1, 473.5, 474.5, 500, DIG. 35

References Cited UNITED STATES PATENTS 8/ 1930 Harnsberger 6/1939 Pulfrich Brazing Manual, published by American Welding Society, Inc., copyright 1963, pp. 66- 89.

Primary Examiner-John F. Campbell Assistant Examiner-Ronald J. Shore Attorney-Ho1man & Stern ABSTRACT: In the connection of a ceramic post to a metal plate, the post is located in a hole in the plate with a porous ring of brazing alloy surrounding the post in contact with the plate. The brazing alloy is then heated to braze the ring to the plate, and the invention resides in the provision of a ring of brazing alloy which has such an initial porosity that when the ring is heated to the brazing temperature the ring sinters to have less porosity and expands less than the post and so will be a tight fit on the post.

PATENTED [15021 lsn 3,625,23

ATT WES CONNECTION OF METALS T CERAMICS This invention relates to the connection of a ceramic post to a metal plate, and in one aspect resides in a method of forming such a connection, comprising locating the post within a hole in the plate, with a ring of brazing alloy surrounding the post in contact with the plate, and then heating the brazing alloy, characterized in that the ring of brazing alloy has a porosity such that when the ring is heated to the brazing temperature the ring will expand less than the post and so will be a tight fit on the post.

In its preferred form, the invention further resides in a ring of brazing alloy for use in a method as specified in the preceding paragraph, characterized in that the porosity of the ring is such that when the ring is heated to the brazing temperature its internal diameter does not increase. Preferably, the internal diameter decreases, but it should be noted that it is within the scope of the method according to the invention for the ring to expand, as long as it expands less than the post.

In the accompanying drawings,

FIGS. 1 and 2 respectively are perspective and side views of an assembly which can be interconnected according to one example of the invention.

Referring to the drawings, it is desired to connect a ceramic post 11 consisting of from 85 percent to 99 percent by weight of alumina and the remainder mainly silicon dioxide, to a stainless steelplate 12. For this purpose the post 11 is located in a hole in the plate 12 and is surrounded by a brazing ring 13 which is a close fit on the post 11 at room temperature. A typical brazing alloy has the following constituents by weight:

Silver 42 percent Copper 18 percent Lead 35 percent Titanium percent In a particular example, the post has a diameter of 089510.001 inch with the hole in the plate of diameter 0915:0010 inch. Using these dimensions the ring must be as close fit on the post as possible, and so allowance must be made for the fact that when the ring is made by pressing in a die, then on removal from the die the ring may change in dimensions. As will be explained later, the forming pressure is important and whatever forming pressure is to be used, the expansion or contraction on relaxation must be previously determined experimentally and allowed for.

In order to form the connection, the ring 13 is heated in a nonoxidizing atmosphere to the brazing temperature of 860 C. in any convenient manner, whereupon the ring 13 is brazed onto the plate 12, and two separate effects determine the gap between the ring 13 and post lll, namely:

1. The ring 13 expands away from the post llll because of the difference in the coefficients of thermal expansion of the two materials. The coefficients of the ceramic and alloy are respectively 7.9 l0- and 20x10 and so on heating from room temperature to 860 C. the post 11 expands from 0.895 inch to 0.901 inch and the internal diameter of the ring 113, assuming an initial diameter of 0.898 inch increases to 0.914 inch.

2. At temperatures above 350 C., sin'tering of the alloy ring 13 will cause contraction of the ring by an amount which is dependent firstly on the porosity of the ring 13 before and after the brazing operation and secondly on the size of the ring. Once the two porosity values are known for a given size ring, the contraction can be calculated.

It will be apparent that in order to make a good joint, the overall expansion of the ring must be less than the expansion of the post. The thermal coefficients of both materials are fixed, and so is the final porosity of the brazing alloy, so that the only variable for a given ring is the initial porosity, which can be controlled to give the desired contraction of the ring relative to the post. The initial porosity depends on the pressure used to form the ring, and decreases with increasing pressure. For the figures quoted above, the maximum forming pressure which can be used is 13.2 t.s.i.., which gives an initial porosify of about 18 percent, which results in a contraction of about .017 inch, which 18 in excess of the expansion of 0.0!6

inch, so ensuring a goodjoint.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. A method of sealingly connecting a ceramic post in an aperture in a metal plate by using a porous ring of a brazing alloy which sinters on heating and reduces in porosity, com prising: choosing an initial porosity for said ring of brazing alloy depending on the expansion of the ceramic post on heat ing; locating the ring of chosen porosity to be a close fit on the post; assembling the ring and the post in said aperture in the metal plate so that the ring is in contact on one side thereof with the metal plate; selectively heating to braze the ring and the metal plate at their surface of contact whereby the porous ring of brazing alloy sinters and reduces in porosity and circumferentially grips the ceramic post to form a vacuumtight seal.

2. The method as claimed in claim 1 wherein the ring is die formed, and the initial porosity of the ring is controlled by altering forming-pressures on the ring in a die during die formation of said ring of brazing alloy. 

1. A method of sealingly connecting a ceramic post in an aperture in a metal plate by using a porous ring of a brazing alloy which sinters on heating and reduces in porosity, comprising: choosing an initial porosity for said ring of brazing alloy depending on the expansion of the ceramic post on heating; locating the ring of chosen porosity to be a close fit on the post; assembling the ring and the post in said aperture in the metal plate so that the ring is in contact on one side thereof with the metal plate; selectively heating to braze the ring and the metal plate at their surface of contact whereby the porous ring of brazing alloy sinters and reduces in porosity and circumferentially grips the ceramic post to form a vacuumtight seal.
 2. The method as claimed in claim 1 wherein the ring is die formed, and the initial porosity of the ring is controlled by altering forming-pressures on the ring in a die during die formation of said ring of brazing alloy. 